1. Field of the Invention
The present invention relates to a V-belt type continuously variable transmission for a straddle-type vehicle.
2. Description of Related Art
In a conventional motorcycle provided with a V-belt type continuously variable transmission, as travel distance is increased, a speed reduction ratio at the time of Top, when a wound diameter of a secondary sheave on a driven side is minimum, is gradually increased due to abrasion of the V-belt, and a maximum speed is consequently lowered. Accordingly, the V-belt needs to be exchanged when it has worn to some extent.
Related art such as WO 2004/044457, for example, discloses a V-belt type continuously variable transmission, in which the exchange time of a V-belt is extended. A V-belt type continuously variable transmission according to related art such as WO 2004/044457 is now explained with reference to FIG. 17.
As shown in FIG. 17, the V-belt type continuously variable transmission (referred below to as CVT) 530 according to the related art comprises a primary sheave 571, a secondary sheave 572, and a V-belt 573 wound around the primary sheave 571 and the secondary sheave 572.
The primary sheave 571 comprises a stationary sheave half 571a, positioned on the right in FIG. 17, and a moving sheave half 571b, positioned on the left in FIG. 17 and opposed to the stationary sheave half 571a. The stationary sheave half 571a is clamped and fixed to a right end of a primary sheave shaft 546c by a lock nut 615 to rotate together with the primary sheave shaft 546c. The moving sheave half 571b rotates together with the primary sheave shaft 546c and is slidable in an axial direction of the primary sheave shaft 546c. A cam surface 611 is formed on the left side of the moving sheave half 571b and a cam plate 612 is arranged on the left of the cam surface 611. A roller weight 613 is arranged between the cam surface 611 and the cam plate 612.
The secondary sheave 572 comprises a stationary sheave half 572a, positioned on the left in FIG. 17, and a moving sheave half 572b, positioned on the right in FIG. 17 and opposed to the stationary sheave half 572a. The moving sheave half 572b is mounted to a right end of a secondary sheave shaft 562. The moving sheave half 572b rotates together with the secondary sheave shaft 562 and is slidable in an axial direction of the secondary sheave shaft 562. The stationary sheave half 572a comprises a substantially doughnut-plate shaped sheave body portion 572c, around which the V-belt 573 is wound, and a substantially cylindrical-shaped boss portion 572d extended toward the right from a right side surface of the sheave body portion 572c. The sheave body portion 572c and the boss portion 572d are fixed together. Also, the boss portion 572d is spline-fitted onto the secondary sheave shaft 562.
A cylindrical-shaped boss 617 fixed to an axial core portion of the moving sheave half 572b is provided on the sheave body portion 572c to be slidable axially of the boss portion 572d. A slide groove 617a is formed on the boss 617 in a slit manner and engaged by a guide pin 618, which is implanted on the boss portion 572d, in a manner to make the same slidable and permit rotation of the boss portion 572d with the boss 617.
A spring bearing member 619 formed from an annular plate is mounted to a tip end of the boss portion 572d of the stationary sheave half 572a by a circlip 619a and a coil spring 620 is interposed between the spring bearing member 619 and the moving sheave half 572b. The secondary sheave shaft 562 and the boss portion 572d of the stationary sheave half 572a are clamped and fixed together by a lock nut 616 provided on the secondary sheave shaft 562.
When an engine (not shown) rotates and the primary sheave shaft 546c is increased in rotation speed, the moving sheave half 571b of the primary sheave 571 is moved toward the stationary sheave half 571a by a centrifugal force of the roller weight 613. Thereby, the primary sheave 571 is gradually increased in wound diameter and the secondary sheave 572 is gradually decreased in wound diameter, with the result that a speed reduction ratio is gradually decreased.
When the primary sheave shaft 546c is further increased in rotation speed, the primary sheave 571 is further increased in wound diameter by a centrifugal force of the roller weight 613 and the moving sheave half 571b of the primary sheave 571 comes to a Top position, in which a spacing between it and the stationary sheave half 571a is minimum. In the Top position, a spacing between the stationary sheave half 572a and the moving sheave half 572b of the secondary sheave 572 is maximum.
FIG. 17 shows a point of time when the moving sheave half 571b of the primary sheave 571 comes to the Top position. As shown in FIG. 17, a right tip end 617b of the boss 617 contacts the spring bearing member 619 in the Top position. Thereby, the moving sheave half 572b on the secondary sheave 572 is restricted to the Top position. In this state, a clearance b is formed between the roller weight 613 and the moving sheave half 571b of the primary sheave 571. At this time, R1 indicates a wound diameter of the primary sheave 571 and R2 indicates a wound diameter of the secondary sheave 572.
While FIG. 17 shows the case where the V-belt 573 is little used, the V-belt 573 wears gradually and a belt width (L) decreases as a travel distance is increased. Then, since the clearance b is formed, the moving sheave half 571b of the primary sheave 571 moves toward the stationary sheave half 571a corresponding to an amount of wear, so that a wound diameter of the primary sheave 571 becomes larger than R1. When the roller weight 613 contacts the moving sheave half 571b and the clearance b disappears, the moving sheave half 571b is restricted to a corrected Top position, in which a spacing between it and the stationary sheave half 571a is smaller than that in the Top position.
On the other hand, since the moving sheave half 572b on the secondary sheave 572 is restricted to the Top position, the V-belt 573 is moved toward an axis and a wound diameter becomes smaller than R2. In this manner, a speed reduction ratio at the time of Top becomes small as compared with that when the V-belt 573 is not used.
When a travel distance is further increased and the V-belt 573 is further decreased in belt width, the V-belt 573 is moved toward an axis and a wound diameter is returned to a magnitude, which is substantially equal to an initial wound diameter R1 since the moving sheave half 571b of the primary sheave 571 is restricted in position. On the other hand, the moving sheave half 572b on the secondary sheave 572 is moved toward the stationary sheave half 572a by the bias of the coil spring 620 corresponding to an amount, by which the V-belt 573 is worn. Thereby, a wound diameter of the secondary sheave 572 returns to a magnitude, which is substantially equal to an initial wound diameter R2. In this manner, a speed reduction ratio at the time of Top becomes substantially equal to that when the V-belt 573 is not used. Thereafter, as a travel distance is increased, a speed reduction ratio at the time of Top is increased.
In this manner, in the V-belt type continuously variable transmission 530 according to the related art, it is possible to extend the travel distance until a speed reduction ratio, at which exchange of the V-belt 573 is needed, is reached, with the result that time for exchange of the V-belt 573 is extended.
However, the related art has a problem in that the maximum speed after the V-belt is exchanged is different from the maximum speed before the V-belt was exchanged. After having earnestly studied the cause for this change in maximum speed, the inventors of the present application have found that the cause resides in collision between the boss 617 and the spring bearing member 619.
That is, since both the boss 617 fixed to the moving sheave half 572b of the secondary sheave 572, and the spring bearing member 619 that supports the coil spring 620, are made of metallic members, they collide many times during running and are consequently shaven and worn. Therefore, the Top position of the moving sheave half 572b varies and is responsible for a change in maximum speed in running after exchange of a V-belt.